Saponins are glycosides widely distributed in plants. Each saponin consists of one or more sugar moeities bonded to a xe2x80x98sapogeninxe2x80x99 aglycon. The sugar moities may be glucose, galactose, or a pentose or methylpentose, while the sapogenin may be a triterpene or a steroid.
Saponins have detergent properties, forming oil-in-water emulsions and producing copious quantities of foam or suds when dissolved in water. The name xe2x80x98saponinxe2x80x99 is derived from the Latin xe2x80x98sapoxe2x80x99 for xe2x80x98soapxe2x80x99. Saponin-containing plants have been employed for centuries as soaps, notably soapwort (Saponaria officialis), soaproot (Chlorogalum pomeridianum), soapbark (Quillaja saponaria), and soapberry (Sapindus mukurossi).
Saponins are employed as foaming agents in soft drinks, frozen carbonated beverages, cocktail mixes, and fire extinguishers. Their detergent properties have resulted in their use in shampoos, facial cleansers, and various cosmetic compositions. Anti-microbial and anti-fungal properties as well as nutraceutical and pharmacological benefits have also been attributed to saponins.
The traditional methods of extracting and isolating saponins from dry vegetable materials consist of extracting with various alcohols (methanol, ethanol, propanol) or water-alcohol mixtures. A defatting step using non-polar organic solvents (i.e. petroleum ether or hexane) may be performed either prior to the extraction step or on the extract itself. Crude saponins are then precipitated by introducing a large volume of acetone or ether. K. Hostettmann and A. Marston, Saponins, Cambridge University Press (1995), pp. 143-145.
The laborious methods used to purify proteins, such as dialysis, ion-exchange chromatography, and size-exclusion chromatography, are often employed to effect further purification of saponins. P. A. Ireland, S. Z. Dziedzic, and M. W. Kearsley, xe2x80x9cSaponin Content of Soya and Some Commercial Soya Products by Means of High-performance Liquid Chromatography of the Sapogenins,xe2x80x9d J Sci. Food Agric. 1986, 37, 694-698.
Soybean seeds (Glycine max, leguminosae) contain about 0.5% by weight saponins. P. A. Ireland, S. Z. Dziedzic, and M. W. Kearsley, xe2x80x9cSaponin Content of Soya and Some Commercial Soya Products by Means of High-performance Liquid Chromatography of the Sapogenins,xe2x80x9d J Sci. Food Agric. 1986, 37, 694-698. Soy saponins have been the subject of investigation since the early 20th century. These compounds consist of a triterpenoid skeleton with various sugar and acetyl moieties. The current consensus is that soyasapogenols A, B, and E are true aglycons, while soyasapogenols C, D, and E are artifacts of hydrolysis that occurs during the process of their isolation. The corresponding glycosides are the so-called xe2x80x98group A saponinsxe2x80x99, xe2x80x98group B saponinsxe2x80x99, and xe2x80x98group E saponinsxe2x80x99, respectively.
Soy saponins have demonstrated anti-mutagenic properties that make them promising agents for cancer prophylaxis. M. J. Plewa et al, xe2x80x9cThe Use of Single Cell Electrophoresis and Flow Cytometry to Identify Antimutagens from Commercial Soybean Products.xe2x80x9d Mutation Research, 402 (1998), 211-218 and Li Baixiang, xe2x80x9cThe study on the Anti-mutation Mechanism of Soyasaponinxe2x80x9d Proceedings of the Third International Soybean Processing and Utilization Conference, (2000), 264-265. Moreover, group B soy saponins have exhibited pronounced suppressive effects on the replication in vitro of the human immunodeficiency virus (HIV). H. Nakashima et al, N. Aids, 3, 10 (1989), 665-668. The chemical structure of soybean saponin is very similar to that of the compound glycyrrhizin, a known anti-viral agent, so soy saponins show promise as building blocks for the synthesis of anti-viral pharmaceutical compounds.
Despite the cultivation and processing of very large quantities of soybeans, at the present time soy saponins are not a significant article of commerce due to the difficulty of isolating and purifying them.
Soy protein concentrates and soy protein isolates are major articles of commerce widely manufactured by extracting hexane-defatted soybean meal with a solvent consisting of a mixture of ethanol and water (typically 75% ethanol by weight) at elevated temperatures. The purpose of this extraction is to separate lecithin, sugars, and complex carbohydrates from the proteins. The ethanol is recovered by fractional distillation, leaving an aqueous solution of sugars (predominantly sucrose, raffinose, and stachyose) and other carbohydrates that also contains isoflavones and saponins. This material is known in the trade as xe2x80x98soy solublesxe2x80x99 or xe2x80x98soy molassesxe2x80x99.
By acidifying soy molasses, typically to a pH of 2.5 to 4.5 at elevated temperatures, isoflavone- and saponin-rich solids are precipitated. These solids can be removed by centrifugation or filtration, then dried to serve as a commercial feedstock for the recovery of soy isoflavones by extraction with suitable organic solvents (i.e. methanol, ethanol, acetone, etc.). U.S. Pat No. 5,919,921.
The subject of the present invention uses acetone/water mixtures at various concentrations and temperatures as a cost-effective method for recovering soy saponins of high purity from soybean-derived materials, while also affording an economical means of recovering soy isoflavones as a by-product.
Although saponins are characteristically insoluble in acetone, mixtures of acetone and water are capable of dissolving saponins at elevated temperatures. The acetone-to-water ratio for this purpose may range from approximately 2.5 to 5.0 parts acetone per one part water and is optimally approximately 4:1 by weight. At appreciably higher concentrations of acetone, the efficiency of extracting saponins is unnecessarily diminished; at appreciably higher concentrations of water, the quantity of saponins recovered may be reduced and undesirable contaminants (chiefly oligosaccharides) are brought into solution, lowering the purity of the saponins that are recovered. Accordingly, while higher concentrations of acetone or water could be used, they might not be as efficient.
To extract the saponins, the feedstock is digested in the optimum acetone/water mixture at acetone reflux temperature (56 degrees C. at atmospheric pressure, or higher temperatures at elevated pressures) in order to bring the saponins into solution. The pH of the mixture should be high enough to bring the saponins into solution. For this purpose it is desirable to use a pH of at least 5.0, and preferably be 6.5 or higher. The undissolved solids are removed by centrifugation, filtration, or settling and decanting while maintaining a temperature greater than 50 degrees C. The centrate or filtrate containing the dissolved saponins is then cooled to a temperature of less than 10 degrees C. (preferably xe2x88x925 to 5 degrees C), whereupon the dissolved soy saponins precipitate and can be removed by centriflugation, filtration, or settling and decanting. The soy isoflavones, which have proven difficult to separate from soy saponins in prior art, remain in solution and are removed in the centrate, filtrate, or decanted supernatant liquid, from which they can in turn be recovered by removing the acetone by fractional distillation and chilling the resulting aqueous pot fraction in order to render the isoflavones insoluble, permitting their recovery by filtration, centrifugation, or settling and decanting.
The ratio of feedstock to the acetone/water mixture can vary. The efficiency of extraction of the isoflavones using a ratio of feedstock to acetone/water mixture of approximately 1 to 10 by mass is typically very high, often exceeding 90%. However, the efficiency of saponin extraction with these parameters is considerably lower (typically 20 to 40%), although the purity of the saponins is quite high. Under these conditions, efficient saponin recovery may require two or more successive extractions with the optimal acetone/water solvent or recourse to counter-current extraction methods that will be obvious to those skilled in the art.
The saponins recovered by filtration, centrifugation, or settling and decanting the chilled wet acetone extract typically have a purity exceeding 70%. They can readily be further purified by dissolving them in a suitable solvent, filtering, and allowing them to crystallize. We have obtained excellent results using a solvent comprised of 70% ethanol/30% water in a ratio of ten to fifteen parts solvent to one part saponins by weight, although those skilled in the art will realize that mixtures of other alcohols and water may prove efficacious. The mixture is heated to 70 degrees C. or higher in order to effect the dissolution of the saponins, filtered at this temperature to remove insoluble impurities, and allowed to cool slowly to 25 degrees C. or less, whereupon saponins with a purity exceeding 95% crystallize and are recovered. Obtaining soy saponins of this purity without recourse to chromatographic techniques has heretofore not been reported.